On aircraft windows for use with internally mounted infrared systems, such as laser instruments and the like, the window must not only provide efficient transmission of the infrared radiation, but must be capable of providing electromagnetic interference (EMI) protection as well. It must also have the structural strength to withstand high aerodynamic loads induced by high speed flight. Furthermore, for velocities above Mach 2 the external skin temperature of the aircraft can rise to over 200.degree. C. At these elevated temperatures the window must retain its optical, EMI protection, and structural properties. It should also have the capability of being deiced.
One approach of providing electromagnetic interference protection and/or deicing for infrared windows is to bond a wire grid with low sheet resistance across the external surface of the substrate. However, when high levels of EMI protection are required, the grid pattern becomes dense and infrared transmission is reduced. The grid also increases scatter and reduces the system modulation transfer function.
Another approach is to apply a conductive film across the glass substrate. This methos is disclosed in U.S. Pat. No. 4,181,774, "Electromagnetic Interference Filter Window" by J. P. Wente, et al. Here, a coating of indium-tin oxide is applied in a ratio of 10/90 having a thickness of 1,000 .ANG.. The coating is applied by a conventional sputtering process and requires annealing at a temperature of 270.degree. C. in the forming gas for one hour, followed by a five-hour anneal in air at 270.degree. C. Thereafter conventional antireflection coatings are applied over both surfaces.
Such a conductive film can obtain 97 percent transmission at 1.06 microns for a sheet resistance of 100 ohms per square. However, this is an expensive process and the sheet resistance value obtained is not low enough for many applicaitons. Furthermore, such a thin conductive coating is easily damaged. Indium Tin Oxide is also strongly absorbing in the medium wave infrared and therefore is not usable for applications which require transmission in such wavelengths.
Another approach is to use an inherently conductive semiconductor as the infrared window substrate. Germanium, for example, is one of the most widely used materials for external infrared windows. Large plates are readily available with diameters of over 12 inches. However, this window material is not suitable for use in high temperature applications due to high absorption losses due to free carrier absorption at elevated temperatures. Gallium Arsenide (GaAs) is another inherently conductive infrared window material. In the paper entitled "Gallium Arsenide Infrared Windows for High-Speed Airborne Applications" by M.J. Brau et al. (SPIE Vol. 29, Emerging Optical Materials, 1981), a 2.8 mm sample which had been compensated with nickel to achieve good infrared transmission provided a sheet resistance of 140 ohms/square. Thicker samples, with the structural strength necessary to absorb aerodynamc loads, at high Mach numbers, would suffer greater transmission losses, particularly at elevated temperatures. In addition, the impurity compensation which is required to reduce inherent conductivity to acceptable levels, is sentitive to both temperatures and doping uniformity.
Accordingly, a primary object of this invention is to provide a material suitable for use as a window material for infrared systems.
Another object of this invention is to provide a window material which is capable of transmitting radiation in the infrared, while simultaneously providing a high level of electromagnetic interference protection.
A further object of this invention is to provide a window material which is capable of transmitting radiation in the infrared and which possesses the requisite strength and stability to withstand the stress and strain encountered within a military operational environment.
A still further object of this invention is to provide an infrared system window material that possesses high transmission of infrared radiation while operating in an environment with temperatures of 200.degree. C. and above.
An additional object of this invention is to provide a window material that is capable of transmitting radiation in the infrared, while simultaneously providing both electromagnetic interference protection and/or deicing capability.